The mandible or inferior maxilla is subject to fractures, dislocation, and tumors. In its composition it is very dense, so that in dividing it a groove should be cut with a saw before the use of the bone-cutting forceps is attempted, otherwise splintering of the bone will ensue. It is the last bone to decay. Its horseshoe shape and exposed position render it unusually liable to fracture. The strongest portion is what one would expect to be the weakest, viz., the symphysis. Its weakest part (or rather the part where it is most often broken) is the region of the mental foramen. The bone is weakened at this point not only by the foramen but also by the deep socket of the canine tooth.

Fig. 71.   Lower jaw of child and adult, showing the mental foramen.

Fig. 71. - Lower jaw of child and adult, showing the mental foramen.

The position of the mental foramen, normally between the two bicuspids (beneath the second in the negro - Humphry), varies in its vertical location between the alveolar border and lower edge of the body, according to age. In infancy it is low down, in young adults it is midway, and in old people it is high up.

The body of the jaw is composed of two parts, one above and one below the external oblique line, which runs from the base of the anterior border of the coronoid process downward and forward to end at the mental tubercle, to one side of the symphysis. The part above this oblique line is the alveolar and the part below is the basal portion of the body.

The mental foramen opens on the oblique line separating the alveolar and basal portions. In early adult life the two portions, basal and alveolar, are about even in size, so that the foramen is below the middle of the jaw. As the teeth are lost the alveolar process atrophies; this naturally leaves the basal portion with the mental foramen on or near its upper surface; therefore, in operating for neuralgia in the aged, if it is desired to attack the mandibular nerve in its canal, it should be searched for near the upper border of the bone.

In infancy the teeth, not having erupted, are contained in the jaw, the alveolar portion is, therefore, large. The basal portion, on the contrary, is quite small, serving merely as a narrow shelf on which the unerupted teeth lie. As the mandibular nerve runs beneath the teeth, the mental foramen is of necessity comparatively low. At birth the condyle is about level with the upper portion of the symphysis, and the body forms with the ramus an angle of 175 degrees. At the end of the fourth year the angle has decreased to about 140 degrees. By adult age the angle has decreased to about 115 degrees, and as the teeth are lost the angle gradually increases until it again reaches 140 degrees.

Temporomandibular Articulation

A knowledge of the movements of the jaw is essential to a proper understanding of the fractures and dislocation's to which it is subject.

Fig. 72.   The temporomandibular articulation.

Fig. 72. - The temporomandibular articulation.

The mandible articulates with the glenoid fossa and its anterior edge or emi-nentia articularis of the temporal bone. Interposed between the condyle below and the bone above, is an interarticular cartilage. This divides the articulation into two portions, an upper and a lower. The ligaments are a capsular, strengthened by an external lateral (temporomandibular) and an internal lateral. The capsular ligament is weakest anteriorly and strongest on the outer side. The thickening of the capsule on its outer side forms the external lateral or temporomandibular ligament. The sphenomandibular or internal lateral ligament is practically distinct from the articulation. It runs from the alar spine on the sphenoid above to the mandibular spine or lingula, just posterior to the mandibular foramen below. Between it and the neck of the bone run the internal maxillary artery and vein. When the condyle glides forward it puts the posterior portion of the capsule on the stretch, and if the jaw is dislocated this part of the capsule is torn. The interarticular cartilage is more intimately connected with the lower portion of the articulation. The same muscle that inserts into the neck of the jaw (the external pterygoid) likewise inserts into the cartilage; therefore, the two move together, so that when the condyle goes forward the cartilage goes forward and rides on the eminentia articularis.

Fig. 73.   External lateral ligament of the lower jaw.

Fig. 73. - External lateral ligament of the lower jaw.